Getting Google to index our sitemap

We have a sitemap on AWS that is retrievable via a url that looks like ours http://sitemap.shipindex.org/sitemap.xml. We have notified Google it exists and it found our 700k urls (we are a database of ship citations with unique urls). However, it will not index them. It has been weeks and nothing. The weird part is that it did do some of them before, it said so, about 26k. Then it said 0. Now that I have redone the sitemap, I can't get google to look at it and I have no idea why. This is really important to us, as we want not just general keywords to find our front page, but we also want specific ship names to show links to us in results. Does anyone have any clues as to how to get Google's attention and index our sitemap? Or even just crawl more of our site? It has done 35k pages crawling, but stopped.

A Sitemap should have not more than 50,000 entrys (Google-News-Sitemaps only 1,000)

and should not be bigger than 50MB

So you have to split it and you allready did.

Now your Main-Sitemap is pointing to other Sitemaps (zipped, but thats not a problem), ok. So whenever GSC is telling me, my Sitemap has errors or no entrys, I open it and check. I did, I just opened the first one, look what is in it:

I opened all of your Sitemaps, no entries in it.
At least, Google indexed >8,000 Pages, but not by Sitemap thats for sure.

You can just create sitemaps with Tools (link at the bottom) or with e.g. screaming frog and upload them to your server (zipped or not doesn't matter) sent to google and done. If your System is not working at the moment, easy workaround for short.
After that - try to find the Bug in creating your sitemaps, solve it and sent these to Google. Before you sent them, open your sitemaps and check if they are working. Don't wait weeks, Google is fast.
List of Sitemap Generators: https://code.google.com/archive/p/sitemap-generators/wikis/SitemapGenerators.wiki

I redid the sitemap and just made them xml, Andreas. It hasn't seemed to help. Still not getting indexed. I don't know where you were seeing that information in the sitemap files. Can you tell me how you opened them to see that? All I see is the normal content.

Now I can see Sitemaps, loadings takes time ... a lot and they look weird, but maybe ok. But there is stuff in it, wich I wont like to have in Google-Index. Northeless - whats the message in GSC?

(opend in Chrome, Firefox and on my pixel as well - the first one is looking good, all linked once had the error, now they are differnet from each other (with Linebreaks or without, with space or without) but contain links at least)

Is the site on a subdomain for pages on a different domain? (didn't saw that) - that makes it way more tricky ...